In an electro-dynamic loudspeaker, the voice coil is described as overhung when the height of the voice coil along a central length axis (vertical axis) of the coil is greater than the height of the magnetic gap in which the coil is positioned to move (in the direction of the length axis). This allows some portion of the coil to always be within the magnetic gap even at maximum excursion (displacement), which produces greater output sound power. The force produced by such a driver however, upon the diaphragm is limited, due to the fact that a majority of the turns (windings) of the coil find themselves outside of the magnetic gap thereby contributing less to the total Lorentz force.
It has been suggested to design a voice coil assembly to have multiple coils, where each coil is electrically independent and is successively positioned along the length axis of the voice coil, where each coil has an axial length that is substantially equal to the height of the magnetic gap. A means for measuring the axial position (along the length axis) of the coil, relative to the magnetic gap, is provided. Such measurements are then used by a control module, which controls audio amplifiers that are coupled to drive the coils, respectively, so that any coil that is least partly present within the magnetic gap is driven with the audio signal, while the coils that are outside of the magnetic gap are not. This may increase power consumption efficiency while producing a Lorentz force that is commensurate with that of the overhung, single coil structure.